Polylactic acid has a feature that it has a higher melting point and is superior in heat resistance compared to other biodegradable resins. However, since polylactic acid has a low melt viscosity, there are problems, for example, that a sufficient expansion ratio cannot be achieved because bubbles are broken upon extrusion foaming and that the thickness of molded articles tends to be uneven because bubbles are not stable in inflation molding or blow molding. Thus, molding conditions are severely restricted. Further, due to its low crystallization rate, polylactic acid has a disadvantage of low production efficiency in injection molding and the like. Therefore, for practical use, it is necessary to improve the melt viscosity, make effective the strain hardening property when measuring elongational viscosity, and to improve the crystallization rate.
Generally, to impart strain hardening properties to a resin composition, a method of adding a polymer of a high polymerization degree or a method of using a polymer containing a long branched chain is considered to be effective. However, in the production of a polymer of a high polymerization degree, polymerization takes a long time, and not only production efficiency is lowered but also coloring or decomposition due to long thermal history is found. For this reason, biodegradable polyester having a weight average molecular weight of, for example, 500,000 or higher, has not been practically used. On the other hand, as the method of producing polylactic acid containing a long branched chain, a method comprising adding a polyfunctional initiator for polymerization is known (JP-A-10-7778, JP-A-2000-136256). However, introduction of branched chain during polymerization involves a problem that discharge of resin is difficult and the degree of branching cannot be arbitrarily modified. Further, methods comprising melt-kneading a layered silicate have also been studied. JP-A-2001-89646 discloses that a resin with a high rigidity and an increased biodegradation rate can be obtained by melt-kneading with a resin a layered clay mineral that has been organized and made to have an average particle size of not more than 1 μm. However, the publication does not contain concrete description of how to adjust the average particle size of the layered clay mineral to not more than 1 μm, and it completely lacks description of molding conditions, and thus whether the moldability was improved or not is unknown.
On the other hand, a method comprising preparing a biodegradable resin and then melt-kneading the same with a peroxide or a reactive compound to be crosslinked, thereby yielding a strain hardening property. This method is variously studied because it is simple and branching can be arbitrarily modified. However, acid anhydride and polycarboxylic acid used in JP-A-11-60928 are not practical because the reactivity tends to be unstable and the pressure must be reduced. In addition, in the case of polyisocyanate used in JP-B-2571329 and JP-A-2000-17037, its molecular weight tends to be decreased upon melting and this causes safety problems in operation. Thus, technology satisfying practical requirements has not yet been established.
JP-A-10-324766 discloses that foaming can be effectively performed when a biodegradable polyester resin synthesized from dibasic acid and glycol is combined and crosslinked with an organic peroxide and a compound containing an unsaturated bond. This method is an example of immersing these crosslinking agents to resin fine particles at a temperature lower than the melting point of the resin, and the case in which divinylbenzene is used as a coagent is described in detail. However, use of (meth)acrylic acid ester compound is not studied, and only application to a biodegradable polyester resin synthesized from dibasic acid and glycol having a low heat resistance has been studied. Further, regarding addition of these crosslinking agent and coagent, no technique for stable and long term operation has been suggested.
A biodegradable polyester with improved heat resistance composed mainly of α- and/or β-hydroxycarboxylic acid unit is also known. However, this biodegradable polyester has a low crystallization rate and is poor in operation ability in molding such as injection molding. To solve this problem, a method of adding inorganic powder has been suggested, but it is not satisfactory.
In addition, gas barrier properties, in particular oxygen barrier property of biodegradable polyester is insufficient, and it was impossible to use biodegradable polyester for applications requiring gas barrier properties, such as food containers. To improve gas barrier properties, a method of dispersing a layered silicate in a resin has been proposed. It is considered that the gas barrier property is improved because permeation of the gas component is blocked by the layered silicate in the resin and the gas moves around the layered silicate. For example, usefulness of layered silicate in polyamide resin is disclosed in JP-B-3284552 and application of layered silicate to aliphatic polyester is disclosed in JP-A-2001-164097. However, rheological characteristics suitable for various molding processes cannot be achieved only by mixing a layered silicate to a resin, and there is a problem of poor operation ability.
The present invention solves the above described problems and provides a biodegradable polyester resin composition excellent in mechanical strength and heat resistance, having a high crystallization rate, causing no trouble in operation, having rheological characteristics advantageously usable in molding of a foamed article or a molded article, and excellent in gas barrier properties, and a process for producing the same and a foamed article and a molded article using the same.